[Support] On Windows, take the affinity mask into account

The number of hardware threads available to a ThreadPool can be limited if setting an affinity mask.
For example:

> start /B /AFFINITY 0xF lld-link.exe ...

Would let LLD only use 4 hyper-threads.

Previously, there was an outstanding issue on Windows Server 2019 on dual-CPU machines, which was preventing from using both CPU sockets. In normal conditions, when no affinity mask was set, ProcessorGroup::AllThreads was different from ProcessorGroup::UsableThreads. The previous code in llvm/lib/Support/Windows/Threading.inc L201 was improperly assuming those two values to be equal, and consequently was limiting the execution to only one CPU socket.

Differential Revision: https://reviews.llvm.org/D92419

1 files changed, 5 insertions, 1 deletions

diff --git a/llvm/lib/Support/Unix/Program.inc b/llvm/lib/Support/Unix/Program.inc
index 8f41fc0..fb56fa4 100644
--- a/llvm/lib/Support/Unix/Program.inc
+++ b/llvm/lib/Support/Unix/Program.inc
@@ -174,7 +174,8 @@ toNullTerminatedCStringArray(ArrayRef<StringRef> Strings, StringSaver &Saver) {
 static bool Execute(ProcessInfo &PI, StringRef Program,
                     ArrayRef<StringRef> Args, Optional<ArrayRef<StringRef>> Env,
                     ArrayRef<Optional<StringRef>> Redirects,
-                    unsigned MemoryLimit, std::string *ErrMsg) {
+                    unsigned MemoryLimit, std::string *ErrMsg,
+                    BitVector *AffinityMask) {
   if (!llvm::sys::fs::exists(Program)) {
     if (ErrMsg)
       *ErrMsg = std::string("Executable \"") + Program.str() +
@@ -182,6 +183,9 @@ static bool Execute(ProcessInfo &PI, StringRef Program,
     return false;
   }
 
+  assert(!AffinityMask && "Starting a process with an affinity mask is "
+                          "currently not supported on Unix!");
+
   BumpPtrAllocator Allocator;
   StringSaver Saver(Allocator);
   std::vector<const char *> ArgVector, EnvVector;